Many industrial fluid flow processes involve the transportation of a high mass fraction of high density, solid materials through a pipe. For example, a process known as hydrotransport is used in many industries to move solids from one point to another. In this process, water is added to the solids and the resulting mixture is pumped through typically large diameter pipes.
Operation of a hydrotransport line typically involves some degree of stratification, where flow velocity near the bottom of the pipe is less than flow velocity near the top of the pipe. The level of stratification in this flow (i.e., the degree of skew in the velocity profile from the top of the pipe to the bottom of the pipe, where the terms “top” and “bottom” and “upper” and “lower” are relative to one another such that a gravitational force vector may be described as extending from the top toward the bottom, or from upper to lower) is dependent on numerous material and process parameters, such as flow rate, density, pipe size, particle size, and the like. If the level of stratification extends to the point where deposition velocity is reached, the solids begin to settle to the bottom of the pipe, and if the condition is undetected and persists, complete blockage of the pipe can occur, resulting in high costs associated with process downtime, clearing of the blockage, and repair of damaged equipment.
To reduce the chance of costly blockage formation, current practice is to operate the pipeline at a flow velocity significantly above the critical deposition velocity. However, this technique has two significant drawbacks due to operating at higher velocities: it causes higher energy usage due to higher friction losses, and it causes higher pipe wear due to abrasion between the solids and the pipe inner surface. This technique may also be undesirable to due high water consumption. A reliable means of measuring parameters such as velocity, level of stratification, and volumetric flow rate of a stratified flow would enable operating the pipeline at a lower velocity, resulting in energy savings and lower pipe wear.
Various technologies exist for measuring physical parameters of an industrial flow process. Such physical parameters may include, for example, volumetric flow rate, composition, consistency, density, and mass flow rate. While existing technologies may be well-suited for aggressive, large diameter flows, these technologies may be unsuitable for stratified flows, which can adversely affect accuracy in measuring physical parameters of the flow.
Several non-commercial techniques for determining the onset of solids deposition in slurry pipelines are described in recent literature. For example, one technique uses a commercial clamp-on ultrasonic flow meter, in Doppler mode, with coded transmissions and cross-correlation detection. The detection point for the meter is set at a certain pipe level, e.g., 10% above the pipe invert (i.e., the pipe bottom for horizontal pipes). Cross-correlation of a time-gated ultrasonic return signal enables detection of reflected signals only from the set point. A decrease in coherence between transmitted and received signals indicates unsteady flow conditions due to solids deposition.
Another existing non-commercial technique measures the apparent electrical resistivity of the slurry near the pipe invert, with a change in resistivity indicating the formation of a solids bed. This technique was deemed to be not very successful due to poor repeatability and other problems.
Another non-commercial technique utilizes self-heating thermal probes mounted in the slurry. A moving slurry removes temperature from the probes, while a stationary solids bed around the probe causes heat to build up. Thus a temperature rise is indicative of solids deposition. While this technique is promising, it is an invasive technique requiring the thermal probes to be placed in the pipe. Such invasive techniques have drawbacks in that they require the process to be stopped to allow for installation and maintenance of the probes.
Another technique involves the installation of a short pipe with slightly larger inside diameter, where a stationary solids bed is allowed to form and is maintained as a control while the main pipeline is operated with no solids bed. The control solids bed is then monitored by one or more of the techniques described above. An increase in the height of the control bed then indicates the likely formation of a sliding bed in the main pipeline, which is a precursor of a stationary bed and eventual blockage. When the control solids bed height increases beyond a certain limit, the flow rate may be increased to avoid solids deposition.
Thus, there remains a need for a method and apparatus for measuring parameters such as velocity, level of stratification, and volumetric flow rate of a stratified flow.